9550f6c16a
The previous commit demonstrates a bug where a commit-graph using
generation v2 could enter a state where one of the GDA2 values has its
most-significant bit set (indicating that its value should be read from
the extended offset table in the GDO2 chunk) without having a GDO2 chunk
to read from.
This results in the following error message being displayed to the
caller:
fatal: commit-graph requires overflow generation data but has none
This bug arises in the following scenario:
- We decide to write a commit-graph using generation number v2, and
decide (correctly) that no GDO2 chunk is necessary (e.g., because
all of the commiter date offsets are no larger than 2^31-1).
- The v2 generation numbers are stored in the `->generation` member of
the commit slab holding `struct commit_graph_data`'s.
- Later on, `load_commit_graph_info()` is called, overwriting the
v2 generation data in the aforementioned slab with any existing v1
generation data.
Then, when the commit-graph code goes to write the GDA2 chunk via
`write_graph_chunk_generation_data()`, we use the overwritten generation
v1 data in a place where we expect to use a v2 generation number:
offset = commit_graph_data_at(c)->generation - c->date;
...because `commit_graph_data_at(c)->generation` used to hold the v2
generation data, but it was overwritten to contain the v1 generation
number via `load_commit_graph_info()`.
If the `offset` computation above overflows the v2 generation number
max, then `write_graph_chunk_generation_data()` will update its count of
large offsets and write the marker accordingly:
if (offset > GENERATION_NUMBER_V2_OFFSET_MAX) {
offset = CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW | num_generation_data_overflows;
num_generation_data_overflows++;
}
and reads will look for the GDO2 chunk containing the overflowing v2
generation number, *after* the commit-graph code decided that no such
chunk was necessary.
The main problem is that the slab containing `struct commit_graph_data`
has a dual purpose. It is used to hold data that we are about to write
to disk while generating a commit-graph, as well as hold data that was
read from an existing commit-graph.
When the two mix, namely when the result of reading the commit-graph has
a side-effect that mixes poorly with an in-progress commit-graph write,
we end up with corrupt data.
A complete fix might be to introduce a new slab that is used exclusively
for writing, and gate access between the two slabs based on context
provided by the caller (e.g., whether this computation is part of a
"read" or "write" operation).
But a more minimal fix addresses the only known path which overwrites
the slab data, which is `compute_bloom_filters()` ->
`get_or_compute_bloom_filter()` -> `load_commit_graph_info()` ->
`fill_commit_graph_info()` by avoiding the last call which clobbers the
data altogether.
This path only needs to learn the graph position of a given commit so
that it can be used in `load_bloom_filter_from_graph()`. By replacing
the last steps of the above with one that records the graph position
into a temporary variable which is then used to load the existing Bloom
data, we eliminate the clobbering, removing the corruption.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission and Documentation/CodingGuidelines).
Those wishing to help with error message, usage and informational message
string translations (localization l10) should see po/README.md
(a po file is a Portable Object file that holds the translations).
To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org (not the Git list). The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks